Hydrogen peroxide is a useful basic chemical indispensable to the food, medicine, pulp, textile and semiconductor industries. Hitherto, hydrogen peroxide has been mass-produced by a continuous synthesis process in which a 2-alkylanthraquinol is autoxidized to obtain the target compound, and the anthraquinone simultaneously obtained is reduced with hydrogen to the original anthraquinone derivative. However, there is a growing desire for an on-site hydrogen peroxide production apparatus. Namely, not only is this a troublesome operation, e.g., repeated rectification is needed for purifying the mass-produced reaction product, but hydrogen peroxide is unstable and is incapable of being stored for long periods of time. Also, an on-site apparatus is desirable from the standpoints of safety in transportation and pollution abatement.
In power plants and factories where seawater is utilized as cooling water, a technique for preventing the attachment of organisms to the inside of a condenser has been employed which comprises directly electrolyzing seawater to generate hypochlorous acid, and utilizing the acid thus generated to inhibit organism attachment. However, restrictions are being placed on the use of hypochlorous acid from the standpoint of environmental conservation. This is intended to prevent the possibility of hypochlorous acid or generated chlorine reacting with marine organisms and organic substances present in the seawater to form organochlorine compounds, which reaction products may cause secondary pollution. On the other hand, it has been reported that the addition of a minute amount of hydrogen peroxide to the cooling water is sufficiently effective in preventing the attachment of organisms. It has further been reported that addition of hydrogen peroxide is effective also in maintaining the quality of water for use in fish breeding farms. However, there are still problems concerning safety in hydrogen peroxide transportation and pollution abatement as discussed above.
Processes for producing hydrogen peroxide through the reduction reaction of oxygen gas have hitherto been proposed. U.S. Pat. No. 3,693,749 discloses several apparatuses for the electrolytic production of hydrogen peroxide, while U.S. Pat. No. 4,384,931 discloses a process for producing an alkaline hydrogen peroxide solution with an ion-exchange membrane. U.S. Pat. No. 3,969,201 proposes a hydrogen peroxide production apparatus having a carbon cathode of a three-dimensional structure and an ion-exchange membrane. However, in these processes, the amount of alkali which must be used to generate hydrogen peroxide increases almost in proportion to the amount of hydrogen peroxide that is to be produced. Consequently, the hydrogen peroxide solution thus obtained has an alkali concentration that is too high relative to the concentration of hydrogen peroxide, and hence is of limited use.
U.S. Pat. Nos. 4,406,758, 4,891,107 and 4,457,953 disclose processes for producing hydrogen peroxide using a porous diaphragm and a hydrophobic carbon cathode. According to each of these patent publications, an aqueous alkaline hydrogen peroxide solution having a low sodium hydroxide/hydrogen peroxide weight ratio can be obtained. In these processes, however, the control of operating conditions is troublesome because the amount of electrolyte solution moving from the anode chamber to the cathode chamber and the rate of movement are difficult to control. In particular, the above processes have a drawback in that hydrogen peroxide cannot be produced in a constant proportion.
Journal of Electrochemical Society, Vol.130, pp. 1117- (1983) proposes a method for stably obtaining an acidic hydrogen peroxide solution in which a cation- and anion-exchange membrane is used and sulfuric acid is supplied to an intermediate chamber. Denki Kagaku, Vol.57, p.1073 (1989), proposes a technique for improving performance by using united membrane electrodes as an anode. Furthermore, Journal of Applied Electrochem., 25 (1995) pp.613-627 describes electrolytic processes for hydrogen peroxide synthesis known at that time. However, these techniques are disadvantageous in cost because the electric power consumption rate is too high, and further have a drawback in that the use and introduction of sulfuric acid is unavoidable. Hence, a fully satisfactory process for hydrogen peroxide production has not yet been developed.
In the aforementioned aqueous hydrogen peroxide solution having an alkali concentration that is too high, the concentration of hydrogen peroxide itself is satisfactory. However, these processes necessitate feeding an alkali ingredient because hydrogen peroxide is efficiently obtained only in an aqueous alkali solution environment, and hence these processes have problems with regard to transportation and safety.
On the other hand, in view of the aforementioned problem associated with direct seawater electrolysis, the use of hydrogen peroxide for seawater treatment is desirable from the standpoints of environmental conservation and economic efficiency, and various investigations are being made thereon. Among such techniques which have been investigated, the use of commercial hydrogen peroxide may give rise to, besides the problems described above, an environmental problem in that the addition of a synthesized chemical which has not been separated from the seawater contaminates the seawater. Also in the case where an alkali is externally supplied for alkali electrolysis for producing hydrogen peroxide, the same problem may arise.
In order to avoid these problems, the present inventors proposed a method comprising subjecting seawater to salt separation to obtain an alkali, obtaining hydrogen peroxide from the alkali, and neutralizing the alkali with an acid separated from the seawater. This method is nearly ideal in that the occurrence of environmental problems can be minimized because there is completely no need to add an external chemical, and in that the amount of electric power required is exceedingly small, although the apparatus therefor is complicated. Consequently, if the above method can be carried out with a simpler apparatus which can be handled more easily, a hydrogen peroxide production process closer to an ideal process can be achieved.